Loopstick antennas



July 1, 1969 I R. w. GILBERT 3,453,634

LOOPSTI CK ANTENNAS Filed May 17. 1966 INVENTOR /Fa:/x zz 1 6715597ATTORN United States Patent 3,453,634 LOOPSTICK ANTENNAS Roswell W.Gilbert, New York, N.Y., assignor to Dictaphone Corporation, Bridgeport,Conn. Filed May 17, 1966, Ser. No. 550,776 Int. Cl. H01q 7/08; H01115/04 US. 'Cl. 343-788 8 Claims ABSTRACT OF THE DISCLOSURE The presentinvention relates, in general, to apparatus for radiating and receivingelectrical signals and, in particular, to improved loopstick antennas.

A loopstick antenna, in its simplest form, includes a core of magneticmaterial, for example, a ferrite rod, around which is wound a coil ofwire. The ends of the coil are connected to a transmitter or a receiver,dependent upon whether the antenna is being used to radiate or receiveelectrical signals.

A desirable feature of loopstick antennas is that they requirerelatively few inexpensive components so that they may be fabricated atrelatively low cost. This renders loopstick antennas particularlyattractive for use in portable radios, wireless intercoms, short-rangetransceivers and other low-cost, wireless radio-frequency units.Loopstick antennas, however, become less effective when constraints areimposed, for example, as to the size, shape and operating frequency.This has, to some extent, limited the use of such antennas in low-costunits such as those set forth above.

A number of techniques have been employed to improve the efliciency of aloopstick antenna, but none of these techniques has proven sufiicientlysatisfactory when such an antenna is to be used in a compact, low-costwireless unit. One approach has been to change the configuration of theantenna, either by way of an increase in its size" or a change in itsshape. If the size of the antenna is increased to improve the etficiencyto the desired extent, the antenna may become so large as to beunsuitable for the compact units under consideration. If the shape ofthe antenna is changed to a more complicated form, this may make it tooexpensive for a low-cost unit.

Another technique which has been employed for increasing the efiiciencyof a loopstick antenna has been to wind the coil around the magneticcore in a special pattern and position. Such special coil position andpattern, however, are complex factors so that winding the coil toachieve the desired efiiciency may be a diflicult and costly operation,again, rendering the antenna unsuitable for compact, low-cost units.

Accordingly, it is an object of the present invention to provide new andimproved loopstick antennas having relatively high efficiencies.

It is another object of the present invention to provide such highlyeflicient loopstick'antennas of relatively small size.

It is a further object of the present invention to provide highlyefiicient and compact loopstick antennas which 3,453,634 Patented July1, 1969 are relatively simple in construction and inexpensive tofabricate.

Briefly stated, these objects are achieved by surrounding the magneticcore of a loopstick antenna with a conductive surface or shield whichforces magnetic flux into a more effective pattern. In one embodiment ofthe invention, this shield is in the form of a cylindrical sheet ofconductive material which is wrapped around the magnetic core. Inanother embodiment of the invention, the conductive surface or shield isin the form of an annular disc encircling the magnetic core intermediateits ends. In still another embodiment of the invention, the cylindricalshield and the annular disc shield are used in combination.

For a better understanding of the present invention, together with otherand further objects thereof, reference is made to the followingdescription, taken in connection with the accompanying drawing, and itsscope will be pointed out in the appended claims.

Referring to the drawing:

FIGURE 1 is a perspective view of one embodiment of a loopstick antennaconstructed in accordance with the present invention;

FIGURE 2 is a horizontal 2-2 of FIGURE 1;

FIGURE 3 is a perspective view of a second embodiment of a loopstickantenna constructed in accordance with the present invention;

FIGURE 4 is a horizontal section taken along line 44 of FIGURE 3; and

FIGURE 5 is a perspective view of a third embodiment of a loopstickantenna constructed in accordance with the present invention.

The loopstick antenna illustrated in FIGURES 1 and 2 includes a core 10of magnetic material and a coil 12 wound around the core 10. The core10, in cylindrical form, may be a rod of the typical ferrite materialwhich is commonly used in loopstick antennas. The ends 12a and 12b ofthe coil 12 are connected to a transmitter or a receiver (neither ofwhich is shown) so that radio frequency signals may be conducted fromthe transmitter to the antenna when the antenna serves to radiatesignals, or radio frequency signals may be conducted from the antenna tothe receiver when the antenna serves to receive signals. For the sake ofclarity, the coil 12 has been shown as having only three turns. It willbe understood, however, that the coil 12 may have many more turnsdependent upon the particular application of the antenna. Among thefactors which determine the number of turns of the coil 12 are,typically, the operating frequency of the antenna and the parameters ofthe circuit to which it is connected.

Located between the core 10 and the coil of wire 12 is a conductivesurface in the form of a sheet or shield of conductive material 14Wrapped cylindrically around the core 10 for one and a fraction turns sothat the fractional turn overlaps a portion of the first turn. In theembodiment illustrated, the overlapped region extends over approximatelyone-quarter of a turn. The cylindrical shield 14 is seen to extend overa substantial portion of the length of the core 10. The shield 14 mayhave such thickness as to be self-supporting, or it may be made of thinfoil material.

It is important that the overlapped portions of the cylindrical shield14 do not make electrical contact So as to form a short-circuited turnaround the core 10. In order to prevent this, a sheet 16 of insulatingmaterial, for example, waxed paper, is wrapped with the sheet ofconductive material 14 around the core 10 so as to be disposed betweenthe two layers of this shield 14 throughout the overlapped region. Inthe drawing, the spaces between the core 10, the shield 14 andinsulating sheet 16 are exaggerated section taken along line simply forpurposes of illustration. It will be appreciated, however, that, inpractice, the conductive sheet 14 and insulating sheet 16 preferably areWrapped tightly around the core in contact with each other.

It is believed that the cylindrical shield 14 forces magnetic flux inthe core 10 into a more effective pattern than would be the case in theabsence of the conductive sheet 14. Without the cylindrical shield 14,the magnetic flux pattern along the core would include appreciable linesof flux which would emerge from and return to the core throughout theintermediate portion of its length. These intermediate flux linescontribute little to a radiated signal and do not contributesignificantly to the current induced in the coil 12 when the antennaserves to receive signals since only a few of the turns of the coilwould be cut by these flux lines.

By providing the conductive cylindrical shield 14, magnetic flux isimpeded in emerging from and returning to the core 10 throughout theintermediate portion thereof within the confines of the cylindricalshield 14. Instead, the magnetic flux lines are forced to followlongitudinal paths in the core 10 so that they emerge from and return tothe core near its ends where the conductive sheet 14 terminates. Thiscauses a concentration of flux in that portion of the flux pattern whichis more effective in the radiation or reception of signals. As a result,the radiation resistance of the antenna is increased, causing a loweringof the Q-factor which, in turn, improves the figure-ofmerit of theantenna. Another way of viewing this result is that the effective polarspacing of the antenna is increased, which is comparable to lengtheningthe antenna. Any closed circular circuit around the core 10 is brokenalong edges 14a and 14b of the cylindrical conductive sheet 14 by theinsulation 16 in the overlapped region so that the net current aroundthe core is zero.

As previously mentioned, the cylindrical conductive sheet 14 may be ofmaterial having such physical thickness as to be self-supporting, or itmay be of foil material. When foil is employed, the cylindrical sheet islikely to require a plurality of turns so that the conductivitythickness of the shield is sufficient to impede flux penetration. Theconductivity thickness of the cylindrical wrap is detenmined by thephysical thickness of the conductive surface and the particular materialselected.

A second embodiment of a loopstick antenna constructed in accordancewith the present invention is illustrated in FIGURES 3 and 4. Thisantenna includes the usual magnetic core 20 and coil 22 wound around thecore. Surrounding the core 20 and the coil 22 is a conductive disc orshield 24 which is spaced slightly from the coil. The disc shield 24 isin the form of an annular plate, with its ends overlapping, which ispositioned near the center of the core 20 and disposed perpendicular tothe longitudinal axis of the core. The overlapping portions of the discshield 24 are spaced apart slightly to prevent electrical contacttherebetween, similar to the insulation between the overlapped layers ofthe cylindrical shield 14 of FIGURES l and 2. The disc shield 24 may besupported in place with respect to the core 20 by any suitable means(not shown) which does not interfere with the radiation or reception ofsignals by the antenna.

The disc shield 24, like the cylindrical shield .14 of FIG- URES 1 and2, forces magnetic flux in the core 10 into a more effective patternthan would be the case in the absence of the shield. Again, those fluxlines which might otherwise contribute little to the effective operationof the antenna are forced to follow almost complete longitudinal pathsthrough the core with the result that there is a maximum concentrationof flux lines in the core. The effect is similar to increasingappreciably the effective crosssectional area of the antenna. Anycircular circuit around the core 20 is broken along the edges 24a and24b of the disc shield 24 by the spacing between its overlapping endportions.

It should be noted that foil material also may be used in the loopstickantenna in FIGURES 3 and 4 instead. of the thicker annular disc 24. Whenusing foil material, a plastic ring, for example, may be employed tosupport the foil.

The spacing between the core 20 and the disc shield.24 is exaggerated inthe drawing simply to illustrate better that the coil 22 passes betweenthe core and the shield and extends to either side of the shield. Inpractice, the shield 24 preferably is snug against the coil 22.

Another embodiment of a loopstick antenna constructed in accordance withthe present invention is illustrated in FIGURE 5. This embodimentcombines the cylindrical shield 14 of the embodiment of FIGURES l and 2,and the annular disc shield 24 of the embodiment of FIGURES 3 and 4.Specifically, the loopstick antenna in FIGURE 5 includes the customarycore 30 and coil 32. Interposed therebetween and surrounding the core isa cylindrical shield 34a of conductive material. Positioned at thecenter of this cylindrical shield 34a and surrounding the coil 32 is anoverlapped annular disc shield 34b.

The cylindrical shield 34a is arranged in a similar manner to thecylindrical shield 14 in FIGURES 1 and 2 in that the cylindrical shield34a also includes one and a fraction turns of conductive materialwrapped around the core 30 with a sheet 36 of insulating materialdisposed between layers of the cylindrical shield 34a throughout theoverlapped region.

The disc shield 34b is arranged in a generally similar manner to thedisc shield 24 in FIGURES 3 and 4, and preferably is positioned snuglyagainst the coil 32. These two annular shields differ, however, in thatthe annular shield 34b is provided with a strip 38 of insulatingmaterial along its inner edge at the gap between its overlappingportions to prevent it from conductively bridging the insulated overlapgap of the cylindrical shield 34a, and to prevent the conductivecylindrical shield 34a from conductively bridging the spaced overlap gapof the disc shield 34b. Thus, the broken circular circuits of thecylindrical shield 34a and the disc shield 34b are not short-circuitedby each other. Hence, there are no closed turns around the core 30 sothat the net current around the core is zero.

The two conductive surfaces, namely, cylindrical shield 34a and discshield 34b, together force the magnetic flux lines into a more effectivepattern in the same manner that they do separately. However, theyprovide a marked improvement over the improvement accomplished by eitherseparately.

In a typical antenna embodying the present invention, in the formillustrated in FIGURES 1 and 2, the length of the core 10 was 4 /2, andits diameter was /2". The material of cylindrical shield 14 was copper,which was wrapped around the core one and a quarter turns. The length ofcylindrical shield 14 was 3" and the thickness was 0.010". Theinsulation 16 was waxed paper and the coil 12 had turns.

While there have been described what are at present considered to be thepreferred embodiments of this invention it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention and it is, therefore, aimedto cover all such changes and modifications as fall within the truespirit and scope of the invention.

What is claimed is:

1. A loopstick antenna comprising: a cylindrical ferrite rod; a sheet ofconductive material and a sheet of insulating material wrapped aroundsaid rod into a cylindrical wrap of more than one turn whereby the edgesof said conductive sheet overlap one another, said sheet of insulatingmaterial insulating portions of said conductive material from each otherover the region of overlap; and a coil of wire wound around saidcylindrical wrap for conducting radio frequency current.

2. A loopstick antenna comprising: an elongated magnetic core, aconductive disc encircling said core and having a non-conductive gap,said disc having a thickness which is small relative to the length ofsaid core, and a width which is of the same order of magnitude as thethickness of said core, said disc extending in a direction which istransverse to the longitudinal axis of said core, and a coil of wirewound around said core for conducting radio frequency current.

3. A loopstick antenna according to claim 2 wherein the conductive discis perpendicular to the longitudinal axis of the core and is positionedat the center of said core.

4. A loopstick antenna comprising:

a cylindrical ferrite rod;

an overlapped annular conductive plate encircling said rod at the centerof said rod, said plate being perpendicular to the longitudinal axis ofsaid rod and overlapping portions of said plate being spaced apart;

and a coil of wire wound around said rod for conducting radio frequencycurrent.

5. A loopstick antenna according to claim 4 wherein the coil of wire onthe rod extends on both sides of the annular plate.

6. A loopstick antenna comprising:

a magnetic core;

a conductive sheet wrapped around said core into a cylindrical shield ofone and a fraction turns whereby said fractional turn overlaps a portionof the first turn;

an insulating material disposed between layers of said conductive sheetover the region of overlap;

a coil of wire wound around said cylindrical shield for conducting radiofrequency current;

and an overlapped annular conductive plate encircling said coil,overlapping portions of said plate being spaced apart.

7. A loopstick antenna according to claim 6 wherein the annular plate isdisposed perpendicular to the longitudinal axis of the core and ispositioned at the center of said core.

8. A loopstick antenna comprising:

a magnetic core;

a conductive sheet wrapped around said core into a cylindrical wrap ofone and a fraction turns whereby said fractional turn overlaps a portionof the first turn;

an insulating material disposed in the gap bet-ween layers of saidconductive sheet over the region of overlap;

a coil of wire wound around said cylindrical wrap for conducting radiofrequency current;

an overlapped annular conductive plate encircling said core, overlappingportions of said plate being spaced apart;

and insulating means interposed between said cylindrical wrap and saidoverlapped annular plate for preventing said plate from conductivelybridging said insulated overlap gap between layers of said cylindricalwrap and for preventing said cylindrical wrap from conductively bridgingthe gap between said spaced overlapping portions of said plate.

References Cited UNITED STATES PATENTS 2,981,945 4/1961 Fyler et al.343787 3,267,478 8/1966 Schiefer 343-788 3,378,626 4/1968 Tucker 336-84FOREIGN PATENTS 468,973 3/1924 Germany.

ELI LIEBERMAN, Primary Examiner.

US. Cl. X.R. 336-84

